Scalar-tensor cosmology for traceless matter field in the large coupling function limit

Scalar tensor (ST) theories of gravitation contain an attractor mechanism towards general relativity. The mechanism is supposed to start back at time of inflation. Consequently the characteristic coupling function of the ST theories could attain a very large value during the radiation epoch. Here ST cosmology in the radiation dominated universe is studied under such situation. A general solution of the scale factor in the radiation dominated flat universe is obtained that is characterized by an additional degree of freedom. An implication of this extra parameter is discussed.